The present invention relates to insulated gate switching devices and more particularly to an insulated gate trench type switching device and a method for the manufacture thereof.
Trench type insulated gate semiconductor devices such as trench type MOSFETs and IGBTs are well known. Typically, a trench type insulated gate semiconductor device includes trenches formed in a semiconductive body, which are lined with a gate insulation layer, such as silicon dioxide, and filled with a conductive gate material such as polysilicon to form a gate structure. The semiconductive body typically includes a first region of a first conductivity type (base region) which extends from a first major surface of the semiconductive body to a predetermined depth. The base region is typically disposed over a second region of a second conductivity type (drift region). The trenches extend from the first major surface of the semiconductive body into the drift region.
In a trench type insulated gate semiconductor device according to a known design, diffused regions of a second conductivity type (conventionally designated as source regions in MOSFETs and as emitters in IGBTs), which extend from the first major surface of the semiconductive body to a predetermined depth, are formed adjacent the sidewalls of the trenches by, for example, an implantation step at a vertical angle to the top surface of the semiconductive body through a mask followed by a diffusion drive. As is well known the diffusion drive step will cause the implanted dopants to travel vertically downward into the semiconductive body and travel in a lateral direction in relation to the sidewalls of the trenches simultaneously.
The diffused regions so formed extend to a depth that is less than the thickness of the base region. Therefore, the diffused regions are spaced from the drift region. As a result a region exists in the base region of the semiconductive body conventionally known as the channel region which is disposed between each diffused region and the drift region and adjacent a sidewall of a trench. It is well known that each channel region may be inverted by application of voltage to the conductive material in the trench adjacent thereto thereby allowing current to flow between the diffused region and the drift region.
The length of the channel region is an important variable as it may control such important design factors as the capability of the device to respond to a high switching frequency. To obtain a shorter channel region in a conventional trench type insulated gate semiconductor device, which may result in having a device that is capable of handling higher switching frequencies, the diffused regions may be driven deeper, thereby shortening the channel region. Such a step, however, requires longer diffusion time at higher temperatures both of which are time consuming and thus increase the manufacturing costs. Additionally, longer diffusion time causes extensive lateral diffusion thus increasing the lateral width of the diffused regions. As a result the lateral area that is covered by the diffused regions increases, which in turn results in lower cell density. In addition, the larger lateral width of each diffused region increases the length of the path that is traveled by leakage or other currents under the diffused regions, thereby adversely affecting the ability of the device to withstand avalanche.
It is, therefore, desirable to have a method for manufacturing a device which may allow for obtaining shorter channel regions and higher cell densities at a reduced cost.